1. Field of the Invention
This invention relates to superconducting oxide articles having improved characteristics for alternating current operation and to a method for manufacturing them. In particular, the invention relates to composite articles comprising multiple substantially electrically decoupled domains, each containing one or more fine filaments of a superconducting oxide material, and to methods and intermediates for manufacturing such composites.
2. Background of the Invention
Since the discovery of the first oxide superconductors less than a decade ago, there has been great interest in developing high temperature superconducting conductors for use in high current applications such as power transmission cables, motors, magnets and energy storage devices. These applications will require wires and tapes with high engineering critical current densities, robust mechanical properties, and long lengths manufacturable at reasonable cost. Superconducting oxide materials alone do not possess the necessary mechanical properties, nor can they be produced efficiently in continuous long lengths. Superconducting oxides have complex, brittle, ceramic-like structures which cannot by themselves be drawn into wires or similar forms using conventional metal-processing methods. Moreover, they are subject to a magnetic effect known as flux jumping which causes sudden localized temperature variations that can force them out of their superconducting state if it is not compensated. Consequently, the more useful forms of high temperature superconducting conductors usually are composite structures in which the superconducting oxides are supported by a matrix material which adds mechanical robustness to the composite and provides good thermal dissipation in the event of flux jumping. The matrix material chosen must be readily formable, have high thermal conductivity, and be sufficiently non-reactive with respect to the superconducting oxides under the conditions of manufacturing and use that the properties of the latter are not degraded in its presence. For composites made by the popular powder-in-tube or PIT process, described, for example, in U.S. Pat. Nos. 4,826,808, and 5,189,009 to Yurek et al. and W. Gao & J. Vander Sande, Superconducting Science and Technology, Vol. 5, pp. 318-326, 1992; C. H. Rosner, M. S. Walker, P. Haldar, and L. R. Motowido, "Status of HTS superconductors: Progress in improving transport critical current densities in HTS Bi-2223 tapes and coils" (presented at conference `Critical Currents in High Tc Superconductors`, Vienna, Austria, April, 1992) and K. Sandhage, G. N. Riley Jr.,. and W. L. Carter, "Critical Issues in the OPIT Processing of High Jc BSCCO Superconductors", Journal of Metals, 43,21,19, all of which are herein incorporated by reference, the matrix material must also provide sufficient oxygen access during manufacturing to allow the formation of a superconducting oxide from its precursor material. Very few matrix materials meet these requirements. Under normal manufacturing conditions, superconducting oxides have adverse reactions with nearly all metals except the noble metals. Thus, silver and other noble metals or noble metal alloys are typically used as matrix materials, and pure silver is the matrix material generally preferred for most high performance applications although composite matrices, including, for example oxide diffusion barriers or silver layers between superconducting oxides and non-noble metals have been suggested in the prior art.
Many of the superconductor applications that have the greatest potential for energy conservation involve operating the superconductor in the presence of an AC magnetic field, or require that the superconductor carry an AC current. In the presence of time-varying magnetic fields or currents, there are a variety of mechanisms that give rise to energy dissipation, hereafter called AC losses, even in superconductors. Thus, the superconductor geometry must be selected to reduce AC losses, in order to preserve the intrinsic advantage of superconductors, the absence of DC electrical resistance. The physics governing AC losses in low temperature superconducting composite materials have been described and analyzed, c.f. Wilson, Superconducting Magnets, Ch 8(1983,1990), W. J. Carr, Jr., AC loss and macroscopic theory of superconductors, Gordon and Breach Science Publishers, New York, 1983, and would be expected to operate in superconducting oxide composites with similar geometries. In general, the primary sources of AC loss are hysteretic loss within the superconducting filament(s),and eddy current loss in the matrix enhanced by coupling between superconducting filament(s). To minimize hysteretic losses, the superconductor is preferably subseparated into many small filaments that are discrete and dimensionally uniform along the length of the conductor. Eddy current losses may be minimized by increasing the electrical resistivity of the matrix or by twisting the filaments, with tighter twist pitches providing lower losses. However, the inherent chemical and mechanical limitations of superconducting oxide composites limit the degree to which these approaches may be relied on for reducing AC losses in high temperature superconducting composites. Conventional methods for increasing the resistivity of the matrix have also been limited. Silver, the matrix material of choice for these composites for the reasons discussed above, has a very low electrical resistivity. Efforts have been made to increase the resistivity of the matrix, for example, by distributing small amounts of oxide-forming metals in finely separated form in a silver matrix, and by using higher resistivity alloys to form all or part of the matrix adjacent to the filaments. However, the presence of even very small amounts of chemically reactive materials near the filament/matrix boundary during the manufacturing process can significantly degrade the properties of the superconducting oxide composite. This is a particularly delicate issue for composites consisting of many fine filaments as the higher surface to volume ratio greatly increases the risk of contamination. In the "PIT" manufacturing process, layers of high resistivity material can also block oxygen access to the filaments during manufacturing, inhibiting the formation of the superconducting oxide from its precursors. In addition, increasing the electrical resistivity of the matrix adjacent to the filaments, whether by surrounding the filaments with a resistive layer or by providing a uniformly doped matrix, generally decreases its thermal conductivity, increasing the risk of flux jumping during use.
Thus, an object of the invention is to provide multifilamentary superconducting composite articles in any desired aspect ratio with improved AC loss characteristics and high critical current densities, and a method for manufacturing them.
Another object of the invention is to provide a method of reducing coupling losses in multifilamentary superconducting oxide composite articles without significantly increasing the risk of contamination of the superconducting filaments by the supporting matrix.
Another object of this invention is to provide a method of manufacturing superconducting composite articles suitable for AC applications which provides adequate oxygen access for formation of a desired superconducting oxide with optimal current carrying capacity.
Another object of the invention is to provide highly aspected multifilamentary BSCCO 2212 and 2223 composite conductors having high current densities, superior AC loss characteristics and robust mechanical properties, and a method for producing them.